This invention is directed to an improved synthetic method for the preparation of etoposide, particularly in the context of improved yield, reduced reaction times and simplified isolation procedures.
Etoposide is an antineoplastic agent having the following structure (1): 
Etoposide has been used effectively as an anti-tumor drug for a variety of conditions. For example, it has been employed in the treatment of acute monocytic leukemia (Schilling""s leukemia), as well as medullary monocytic leukemia, and has proved effective for the treatment of recticulum cell sarcoma, tissue-cellular lymphoma, lymphasarcoma and Hodgkin""s disease. Due to the well recognized activity of etoposide, a number of techniques have been developed directed to its synthesis.
One synthetic technique is that reported by Kuhn et al. in Swiss Patent No. 514,578, and related techniques disclosed in U.S. Pat. Nos. 3,408,411 and 3,524,844. Kuhn et al. discloses the preparation of etoposide by the reaction of 4xe2x80x2-demethyl-epipodophyllotoxin (2) with chloroformic acid benzyl ester (as a protecting group for the 4xe2x80x2-phenolic alcohol) to give 4xe2x80x2-carbobenzoxy-4xe2x80x2-demethyl-epipodophyllotoxin (3), followed by reaction of (3) with 2,3,4,6-tetra-O-acetyl-xcex2-D-glucose (4) in the presence of boron trifluoride diethyl etherate to give tetra-O-acetyl4xe2x80x2-carbobenzoxy-4xe2x80x2-demethyl-epipodophyllotoxin-xcex2-D-glucoside (5): 
The carbobenzoxy protecting group of compound (5) is removed to give tetra-O-acetyl-4xe2x80x2-demethyl-epipodophyllotoxin-xcex2-D-glucoside (6), which is then deacylated in the presence of zinc acetate to form 4xe2x80x2-demethyl-epipodophyllotoxin-xcex2-D-glucoside (7): 
Conversion of 4xe2x80x2-demethyl-epipodophyllotoxin-xcex2-D-glucoside (7) to etoposide is achieved by reacting with acetaldehyde-dimethylacetal and p-toluene sulphonic acid. This synthetic method, however, in addition to requiring numerous reaction steps, is of low yield. That is, only about 18% etoposide based on 4xe2x80x2-demethyl-epipodophyllotoxin (2).
Another synthetic method is disclosed by Kurabayashi and Kalsuhiko et al. in Japanese Patent No. 84/98098. Unlike the method of Kuhn et al., 2,3-di-O-chloroacetyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose (8)xe2x80x94as opposed to glucose (4) of Kuhn et al.xe2x80x94is reacted directly with a 4xe2x80x2-(protected)-4xe2x80x2demethyl-epipodophyllotoxin (9) in the presence of boron trifluoride etherate to give intermediate (10): 
The resulting intermediate (10) is then converted to etoposide by reaction with zinc acetate. While this method represents an improvement of the technique of Kuhn et al., strict reaction conditions are required for controlling monoacylation of the 4xe2x80x2-phenolic hydroxyl group to generate compound (9) from 4xe2x80x2-demethyl-epipodophyllotoxin.
A further improvement to the synthesis of etoposide is disclosed in U.S. Pat. No. 5,206,350 by Wang et al. In that method, direct addition of 2,3-di-O-chloroacetyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose (8) to 4xe2x80x2-demethyl-epipodophyllotoxin (2) is achieved in the presence of boron trifluoride etherate as catalyst without having to employ a protecting group on the 4xe2x80x2-phenolic hydroxyl group, giving 4xe2x80x2-demethylepipodophyllotoxin4-(2,3-di-O-chloroacetyl-4,6-O-ethylidene)-xcex2-D-glucopyranoside (11): 
After removal of the chloroacetyl protecting groups from compound (11) with zinc acetate in methanol, etoposide is obtained at a reported yield of 54% (based on 4xe2x80x2-demethyl-epipodophyllotoxin).
While eliminating the need to protect the 4xe2x80x2-phenolic hydroxyl group of compound (2), Wang et al. still suffers drawbacks with regard to yield, extended reaction times and isolation methodology. Accordingly, there exists a need in the art for improved synthetic methods for making etoposide which overcome these deficiencies. The present invention fulfills these needs and provides further related advantages.
This invention is directed to an improved synthetic procedure for the synthesis of etoposide. Due to the well know utility of etoposide, particularly in the context of cancer treatment, synthetic methods which result in higher yields of etoposide are of particular interest, especially with regard to commercial manufacture of the same. The present invention provides a relatively simple method of making etoposide at higher yields than existing techniques, and under more favorable reaction times and much simplified isolation procedures.
In one embodiment, a method for making etoposide is disclosed comprising the steps of:
condensing 4xe2x80x2demethyl-epipodophyllotoxin of formula (2) with a glucopyranose of formula (13) in an organic solvent at a temperature below xe2x88x9230xc2x0 C. and in the presence of trimethylsilyl triflate catalyst to give a compound of formula (14): 
where R1 is xe2x80x94COCH3, xe2x80x94COCH2X, xe2x80x94COCHX2 or xe2x80x94COCX3, and each occurrence of X is independently selected from a halogen; and
converting compound (14) to etoposide (1) having the following formula: 
In more specific aspects of this embodiment, the compound of formula (13) is present in about 1.5 to about 2.0 equivalents based on the compound of formula (2), and trimethylsilyl triflate is present in about 1.5 to about 2.5 equivalents based on the compound of formula (2). Condensation of the compound of formula (13) and the compound of formula (2) is typically in the range from xe2x88x9240xc2x0 C. to xe2x88x9260xc2x0 C., and may be performed in the presence of a drying agent such as dry molecular sieve or zeolite. The organic solvent is typically a halogenated or non-halogenated organic solvent, including (but not limited to) acetonitrile, acetone, diethylether, chloroform, dichloromethane, 1,2-dichloroethane, or mixtures thereof. Preferred R, groups of the glucopyranose of formula (3) include xe2x80x94COCHCl2 and xe2x80x94COCH2Cl.
In the condensing step, the trimethylsilyl triflate may be added to the mixture of the compound of formula (13) and the compound of formula (2) over a period of about 30 minutes, with the temperature of the mixture being maintained at about xe2x88x9250xc2x0 C. to about xe2x88x9240xc2x0 C. The condensing step may be completed in about 1 to 2 hours.
In the converting step, such conversion may be accomplished by alcoholysis with, for example, a transesterification catalyst such as zinc acetate dihydrate. The zinc acetate dihydrate may be present in about 1.0 to about 2.0 equivalents based on the compound of formula (14). The converting step may be performed in the presence of an organic solvent, including (but not limited to) a C1-4alkanol such as methanol. The compound of formula (14) and zinc acetate dihydrate may be heated to a temperature ranging from about 60xc2x0 C. to about 75xc2x0 C. for up to about 2 hours.
In a further embodiment, compound (14) may be eluted through a celite/basic alumina column, or a silica gel, prior to being converted to etoposide. In still a further embodiment, the resulting etoposide may be purified. Such purification may be accomplished by, for example, crystallization, extraction or column chromatography. Crystallization may be from a C1-4alkanol, a C1-4aliphatic ester, or a non-polar solvent, where the C1-4alkanol includes methanol and ethanol, the C1-4aliphatic ester includes ethyl acetate, and the non-polar solvent includes n-pentane or hexanes or petroleum ether. The temperature of such crystallization may be from xe2x88x924xc2x0 C. to 0xc2x0 C. for 8 to 12 hours.
Preferably, the etoposide of this invention is at least 99% pure, is substantially free of a dimer of 4xe2x80x2demethyl-4-epipodophyllotoxin, and is substantially free of etoposide in the xcex1-glucoside form. In this context, the term xe2x80x9csubstantiallyxe2x80x9d means less than 0.5% by weight.
In another embodiment of this invention, a method for making etoposide is disclosed comprising the steps of:
condensing 4xe2x80x2-demethyl-epipodophyllotoxin of formula (2) with 2,3-di-O-dihaloacetyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose of formula (13) in an organic solvent at a temperature below xe2x88x9220xc2x0 C. and in the presence of a Lewis acid catalyst to give 4xe2x80x2-demethylepipodophyllotoxin-4-(2,3-di-O-dihaloacetyl-4,6-O-ethyidene)-xcex2-D-glucopyranoside of formula (14): 
where R1 is xe2x80x94COCHX2 and each occurrence of X is independently selected from a halogen; and
converting the 4xe2x80x2-demethylepipodophyllotoxin-4-(2,3-di-O-dihaloacetyl-4,6-O-ethylidene)-xcex2-D-glucopyranoside of formula (14) to etoposide (1) having the following formula: 
In more specific aspects of this embodiment, the Lewis acid may be a tri(C1-4alkyl)silyltrifluoromethane sulfonate, such as trimethylsilyl triflate, or a boron trifluoride di-C1-4alkylether complex, such as boron trifluoride etherate. Further Lewis acids include (but are not limited to) ZnCl2, DEAC, CF3SO3H or CF3SO3Ag. A preferred R1 for the 2,3-di-O-dihaloacetyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose of formula (13) is xe2x80x94COCHCl2.
In still a further embodiment of this invention, a method for making etoposide is disclosed comprising the steps of:
condensing 4xe2x80x2-demethyl-epipodophyllotoxin of formula (2) with a glucopyranose of formula (13) in an organic solvent at a temperature below xe2x88x9220xc2x0 C. and in the presence of a Lewis acid catalyst to give a compound of formula (14): 
xe2x80x83where R1 is xe2x80x94COCH3, xe2x80x94COCH2X, xe2x80x94COCHX2, or xe2x80x94COCX3, and each occurrence of X is independently selected from a halogen;
collecting compound (14) by elution through a celite/basic alumina column or silica gel; and
converting the collected compound (14) to etoposide (1) having the following formula: 
In more specific aspects of this embodiment, Lewis acid is a tri(C1-4alkyl)silyltrifluoromethane sulfonate, such as trimethylsilyl triflate, or a boron trifluoride di-C1-4alkylether complex, such as boron trifluoride etherate. Further Lewis acids include (but are not limited to) ZnCl2, DEAC, CF3SO3H or CF3SO3Ag. A preferred R1 for the glucopyranose of formula (13) is xe2x80x94COCHCl2.
These and other aspects of this invention will be evident upon reference to the following detailed description.
As mentioned above, this invention is directed to a method for making etoposide at high yield and under simplified reaction conditions. The method involves the direct condensation of 4xe2x80x2-demethyl-epipodophyllotoxin (2) with 2,3-di-O-dichloroacetyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose (13) in the presence of trimethylsilyl trifluoromethane sulfonate (TIMSOTf) to yield 4xe2x80x2-demethylepipodophylotoxin-4-(2,3-di-O-dichloroacetyl-4,6-O-ethylidene)-xcex2-D-glucopyranoside (14) as represented by the following Reaction Scheme 1: 
The above reaction is carried out at a temperature range of below xe2x88x9230xc2x0 C. and generally in the range of xe2x88x9240xc2x0 C. to xe2x88x9250xc2x0 C., for a period of time ranging from 1 to 3 hours and typically from 1 to 2 hours. As with the prior technique of Wang et al., the condensation is performed without protecting the 4xe2x80x2-phenolic hydroxyl moiety of compound (2). However, unlike the prior technique, reaction product (14) may be collected by filtration through basic alumina, thereby avoiding the long isolation procedures reported by Wang et al. and Kuhn et al. For example, Kuhn et al. utilizes aqueous base treatment followed by repeated extractions with an organic solvent, and successive washings with hydrochloric acid solution, NaHCO3, water and drying over anhydrous sodium sulfate. Such a long work-up methodology generally leads to the formation of undesired side-products, which are avoided in the simplified isolation procedures of the present invention.
Regeneration of the alcoholic group at the 2- and 3-positions of the glycosidic moiety of compound (14) may be accomplished by alcoholysis using zinc acetate dihydrate. The resulting product of this transesterification reaction is etoposide (1).
In Reaction Scheme 1 above, 4xe2x80x2-demethyl-epipodophylloxin (2) may be obtained from podophyllotoxin by, for example, the techniques disclosed in U.S. Pat. No. 3,524,844 to Kuhn et al. (incorporated herein by reference). Further, 2,3-di-O-dichloroacetyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose (13) may be prepared from 2,3-di-O-dichloroacetyl-1-O-benzyloxycarbonyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose (12) by hydrogenolysis using 10% palladium on activated carbon.
Thus, the overall reaction for synthesis of etoposide by the method of this invention may be represented by the following Reaction Scheme 2: 
In an alternative embodiment of this invention, the R1 groups of glucopyranose (13) may be xe2x80x94COCH3, xe2x80x94COCH2X, xe2x80x94COCHX2 or xe2x80x94COCX3, where each occurrence of X is independently selected from a halogen. In a preferred embodiment, as represented by Reaction Schemes 1 and 2 above, R1 is xe2x80x94COCHCl2 However, in other embodiments suitable R1 moieties include xe2x80x94COCH2Cl and xe2x80x94COCH2Br, as well as the other R1 moieties noted above.
A number of advantages are associated with the present invention. For example, this synthetic technique is a highly efficient process, wherein all reactions may be carried out within 1-2 hours, and providing much simplified isolation procedures. Protection of the 4xe2x80x2-phenolic group of starting compound (2) is avoided, and no purification is involved in the individual steps, with only the final product being re-crystallized once. Further, all reactions are readily monitored by thin layer chromatography. Lastly, and perhaps most importantly, the overall yield is significantly higher that existing synthetic techniques.
Presently, the best synthetic method for making etoposide is the technique disclosed above by Wang et al. In that technique, the overall yield of etoposidexe2x80x94calculated based on compound (2)xe2x80x94was 54%. In contrast, utilizing trimethylsilyl triflate as the catalyst, the overall yield of etoposide according to the method of the present invention is about 68%xe2x80x94again, based on compound (2). This represents a 25% increase in yield over Wang et al. Furthermore, the present invention provides new isolation conditions for the intermediate (14), thus eliminating the formation of further side-products and any epimerized side-products.
It should be recognized that the technique of Wang et al. employs boron trifluoride etherate as the catalyst for formation of 4xe2x80x2-demethylepipodophyllotoxin-4-(2,3-di-O-chloroacetyl-4,6-O-ethylidene)-xcex2-D-glucopyranoside (11) by the direct condensation 2,3-di-O-chloroacetyl-(4,6-O-ethylidene)-xcex2-D-glucopyranose (8) with 4xe2x80x2-demethyl-epipodophyllotoxin (2). While boron trifluoride etherate of Wang et al. and others, and trimethylsilyl trifluoromethane sulfonate of the present invention may both be classified as Lewis acids, it has been surprisingly found that trimethylsilyl trifluoromethane sulfonate works significantly better than boron trifluoride etherate, as well as significantly better than other Lewis acids tested, including DEAC, ZnCl2, CF3SO3H and CF3SO3Ag.
While not intending to be limited by the following, it is believed that trimethylsilyl trifluoromethane sulfonate may function by a different mechanism than that of boron trifluoride etherate. The latter is believed to generate a carbocation intermediate at the C4 position of the aglucone, which is then attacked by the free hydroxyl group of the glucopyranose. In contrast, trimethylsilyl triflate apparently reacts with the free hydroxyl group of the glucopyranose to form a good leaving group, which facilitates the later attachment of the lignan moiety to give compound (14) with retention of stereochemistry that is the same as that of the starting materials. Furthermore, the use of trimethylsilyl triflate allows for a more efficient coupling, which results in 80% yield from lignan (2) to compound (14)xe2x80x94based on the lignan (2)xe2x80x94which is achieved within 2 hours, as opposed to only about 60% yield by Wang et al. Therefore, by use of trimethylsilyl triflate, significant advantages are achieved, including increased yield and reduced reaction times.
Furthermore, when the present invention employs boron trifluoride etherate as catalyst (the same catalyst disclosed by Wang et al.), improved yields are again obtained, that is, 60% yield compared to the 54% of Wang et al. This difference in yield is believed due to the use of a different glucopyranose (where R1 is xe2x80x94COCHCl2, rather than xe2x80x94COCH2Cl of Wang et al.), as well as being attributable to the simplified isolation procedures of this invention.
In addition to trimethylsilyl trifluoromethane sulfonate and boron trifluoride etherate, other. Lewis acids may, be employed, provide compound (14 is collected by elution through a celite/basic alumina column or a silica gel prior to converting the collected compound (14) to etoposide (1).